Recently, the environmental movement has been active, and the exhaust regulations of exhaust gas such as carbon dioxide gas that causes the global warming are becoming more rigorous. Thus, in the car industry, electric vehicles (EVs) and hybrid electric vehicles (HEVs) have been actively developed in place of automobiles using fossil fuels such as gasoline, diesel oil, and natural gas. A nickel-hydrogen secondary battery or a lithium ion secondary battery is used as the batteries for such EVs and HEVs. Recently, nonaqueous electrolyte secondary batteries such as a lithium ion secondary battery have been often used because these provide a battery with lightweight and high capacity.
The batteries for EVs and HEVs are required not only to be environmentally friendly but also to achieve the basic performance of automobiles, that is, highly developed traveling performance such as acceleration performance and hill-climbing performance. In order to achieve such requests, the battery is required to have both an increased capacity and high power output. Generally, for the nonaqueous electrolyte secondary battery for EVs and HEVs, a prismatic sealed battery that includes an electric power-generating element in a prismatic outer can made of an aluminum-based metal is often used. However, the internal resistance of the battery is required to be reduced as much as possible because a large current flows in the battery when it is discharged at high power. Thus, various improvements have been achieved in order to prevent welding defects between the substrate of the electrode sheet and the collector member to lower the internal resistance in the electric power-generating element of the battery.
Examples of the method for electrically connecting the substrate of the electrode sheet and the collector member in the electric power-generating element to collect electrical current include a mechanical crimping method and welding method. Among them, the welding method is suitable for collecting electric current in a battery that requires to have a high power output because the resistance is readily lowered as well as the weld is less changed with time. Furthermore, in order to lower the resistance in the lithium ion secondary battery, aluminum or an aluminum alloy is used as the materials for the substrate of the positive electrode sheet and collector member, and copper or a copper alloy is used as the materials for the substrate of the negative electrode sheet and collector member. However, characteristics of the aluminum, aluminum alloy, copper, and copper alloy include a small electric resistance and large thermal conductivity, requiring a very large amount of energy is required for the welding.
The following methods are conventionally known as welding methods used between the substrate of the electrode sheet and the collector member in the electric power-generating element.
(1) Laser welding method (see JP-A-2001-160387)
(2) Ultrasonic welding method (see JP-A-2007-053002)
(3) Resistance welding method (see JP-A-2006-310254)
In the laser welding method, a high-energy laser beam is required because the aluminum, aluminum alloy, copper, and copper alloy that are the metals to be welded have a high reflectivity of about 90% with respect to the YAG (yttrium-aluminum-garnet) laser beam widely used for metal welding. Furthermore, the laser welding method presents other issues such as when the aluminum, aluminum alloy, copper, or copper alloy is laser-welded, the weldability greatly varies depending on the surface conditions, and the occurrence of spattering is unavoidable, similar to laser welding of other materials.
In the ultrasonic welding, a large amount of energy is also required because the aluminum, aluminum alloy, copper, and copper alloy that are the metals to be welded have large thermal conductivity, and furthermore, the positive electrode active material and negative electrode active material can possibly fall off due to the ultrasonic vibration during welding. Thus, in the invention disclosed in JP-A-2007-053002, the electrode assembly as the electric power-generating element is compressed during ultrasonic welding so that the dropped active material does not fall into the electrode assembly.
In addition, the resistance welding presents issues such as a high current is required to be input in a short period because the aluminum, aluminum alloy, copper, and copper alloy as the metals to be welded have a small electric resistance and large thermal conductivity, the electrode rod for resistance welding and the collector member are sometimes melted together during resistance welding, and melting or spark occurs at locations other than the welded part.
As discussed above, though each of the three welding methods has advantages and disadvantages, considering productivity and economy, the resistance welding method that is widely used as a conventional method for welding metals will preferably be employed. However, the electrode assembly for a lithium ion secondary battery and the like for EVs and HEVs has a structure in which the positive electrode sheet and the negative electrode sheet are wound or stacked with a separator interposed therebetween. Then, the substrate exposed portions of the positive electrode sheet are placed on one side and those of the negative electrode sheet are placed on the other side, then the substrate exposed portions of the positive electrode sheet are stacked and welded to the positive electrode collector member, and the substrate exposed portions of the negative electrode sheet are stacked and welded to the negative electrode collector member. When the lithium ion secondary battery and the like for EVs and HEVs has a large capacity, the stacking amount of each of the positive electrode substrate exposed portions and negative electrode substrate exposed portions becomes very large.
Thus, a great deal of welding energy is required in order to reliably resistance-weld each of the collector members made of aluminum or an aluminum alloy to the substrate exposed portions of the positive electrode sheet, and the collector members made of copper or a copper alloy to the substrate exposed portions of the negative electrode sheet. Moreover, when a large welding energy is applied during resistance welding, the amount of spattered particles increases thereby increasing the possibility that the particles migrate to the electrode assembly resulting in an internal short circuit.
In contrast, JP-A-2003-249423 discloses the storage element in which, in the electrode assembly in which the positive electrode sheet and the negative electrode sheet are flatly wound with a separator interposed therebetween, each of the substrate exposed portions of the electrodes is divided into two and welded to the collector member in order to reduce each width of the substrate exposed portions, which are extended from the separator, of the electrodes. Hereinafter, the structure of the storage element disclosed in JP-A-2003-249423 will be described with reference to FIGS. 8A to 8C and 9. FIG. 8A is a cross-sectional view showing an electric double layer capacitor as the storage element disclosed in JP-A-2003-249423, FIG. 8B is a cross-sectional view taken along the line VIIIB-VIIIB of FIG. 8A, and FIG. 8C is a cross-sectional view taken along the line VIIIC-VIIIC of FIG. 8A. Furthermore, FIG. 9 is a diagram showing a welding process between the substrate exposed portion of the electrode and the collector member in FIGS. 8A to 8C.
As shown in FIGS. 8A to 8C, the storage element 50 includes a wound electrode assembly 51 in which a positive electrode sheet and a negative electrode sheet are flatly wound with a separator interposed therebetween (not shown), and the wound electrode assembly 51 is placed in a prismatic aluminum outer can 52. Furthermore, a positive electrode collector member 53a and a negative electrode collector member 53b of the storage element 50 include U-shaped wing members 54a and 54b on one end, respectively. The U-shaped wing members 54a and 54b are connected to the substrate exposed portion 55a of the positive electrode sheet and the substrate exposed portion 55b of the negative electrode sheet, respectively. The other end of each of the collector members is connected to a positive electrode terminal 56a or a negative electrode terminal 56b. In the storage element 50, the substrate exposed portions 55a of the positive electrode sheet are gathered and divided into two, and each is welded to each of the two sites on the outer side of the U-shaped wing member 54a, as well as the substrate exposed portions 55b of the negative electrode sheet are divided into two, and each is welded to each of the two sites on the outer side of the U-shaped wing member 54b. 
The welding is performed by ultrasonic welding as described below. For example, for the positive electrode sheet, as shown in FIG. 9, one of the two-divided substrate exposed portions 55a of the positive electrode sheet is placed on the outer side of the U-shaped wing member 54a, a horn 57 of an ultrasonic welding equipment (not shown) is brought into contact with the outer surface of the substrate exposed portion 55a, an anvil 58 is placed on the inner side of the U-shaped wing member 54a, and then ultrasonic welding is performed. Similar ultrasonic-welding is performed on the other two-divided substrate exposed portion 55a of the positive electrode sheet and on the negative electrode sheet.
The invention disclosed in JP-A-2003-249423 provides the effect of good volumetric efficiency of the storage apparatus because the exposed width of both the positive electrode substrate exposed portion and the negative electrode substrate exposed portions can be reduced. However, the invention presents an issue in being a complex fabrication apparatus. For example, multiple welding steps are required in order to weld the positive electrode collector member or the negative electrode collector member to the positive electrode sheet or the negative electrode sheet, respectively, as well as opening spaces being required at the central part of the wound electrode assembly in order to place each U-shaped wing member of the positive electrode collector member and the negative electrode collector member, and an anvil is required to be placed inside of the U-shaped wing member during ultrasonic welding.
Furthermore, JP-A-2003-249423 describes that the ultrasonic welding method is specifically preferably employed in the step for connecting the electrode sheet. However, in Example, the number of windings is 16 (eight for one of the two-divided sheets) and the stacking thickness is 320 μm. In contrast, in a large capacity sealed battery such as the lithium ion secondary battery for EVs and HEVs, the number of each of the stacked positive and negative electrode substrate exposed portions is far larger than in the case of the invention disclosed in JP-A-2003-249423 and the stacking thickness is far larger.
Thus, in order to stably weld the stacked positive and negative electrode substrate exposed portions to the collector members by the ultrasonic welding method, a large capacity sealed battery such as the lithium ion secondary battery for EVs and HEVs requires a high pressure for closely contacting each of the stacked positive and negative electrode substrate exposed portions to the collector members, and a large amount of energy is required so that the ultrasonic vibration reaches the other end of each of the stacked positive and negative electrode substrate exposed portions. In the invention disclosed in JP-A-2003-249423, because the anvil placed inside the U-shaped collector member has to receive the pressure and the ultrasonic wave energy, the anvil requires a corresponding rigidity. Moreover, it is technically very difficult to find a condition in which the welding is stably performed both under high pressure and using an anvil of a size capable of being inserted inside the U-shaped collector member.
On the other hand, when the two-divided positive electrode sheets or negative electrode sheets are resistance-welded, a method of separately welding each of the divided sheets and a method of series spot welding for simultaneously welding the divided sheets are discussed. Here, the series spot welding is preferred for reducing the amount of welding repetitions. As shown in FIG. 10, when members to be welded 73 and 74 are welded at two points on the same axis as that of a pair of electrode rods 71 and 72 for resistance welding, a conventional series spot welding art mainly employs a method in which an U-shaped welding member 75 is interposed therebetween to weld the upper and lower parts of the U-shaped welding member 75. The method is widely used because the U-shaped welding member 75 is readily prepared from a metal plate and a projection for easy and stabilized resistance welding is readily prepared. However, the method has an issue in requiring complex welding equipment. For example, a pressure receiver 76 is required inside the U-shaped welding member 75 in order to prevent the deformation of the U-shaped welding member by the pressure from the welding electrode rods 71 and 72, and a metal block is required for carrying current.